Harmonic-noise-suppression device



Oct. 14, 1930. w. P. MASON 1,778,035

HARMONIC NOISE SUPPRESSION DEVICE Filed June 28, 1929 2 Sheets-Sheet 1 A T TOHNE Y ORDINATE6 0F CURVE Ll Oct. 14, 1930.

TRANSMISSION L066 DECDBELS N O O o W. F. MASON HARP/IONIC NOISE SUPPRESSION DEVICE Filed June 28, 1929 2 Sheets-Sheet 2 Fla. 2

R.M.6. PRESSURE m ARBITRARY umrs ORDINATE6 or CURVE b woo I860 I920 I960 2040 2:00 FREQUENCY- CYCLES PER SECOND W VE/v 70/? W F. MA sorv Patented Oct. l4, 1930 unnan STATES PATENT OFFICE WARREN P. MASON, OF EAST ORANGE, NEW JERSEY, ASSIGNOB TO BELL TELMDHI LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION 01 NEW YORK HABIONIO-NOISE-SUPPBESS ION DEVICE Application filed June 28,1929. Serial No. 374,585.

This'invention relates to wave transmission,fand more particularly to means for separating sustained noises from speech in a telephone line. 1

An object of the invention is to obtain improved selectivity in the separation of interfering noisecurrents from speech waves in telephone systems. i

Another object is to improve the operation of voice operated relays in speech transmission systems.

In tele hone systems subject to interference arising rom power conductors the interference currents have frequencies which in general occur in a fairly well defined harmonic order- These harmonically related currents produce sustained noises in telephone receivers which cause difficulty in interpreting speech and in addition, they impair the operation of voice-operated relays in systems where devices of this sort are used. Since the interfering noise currents occur in harmonic order, and ingeneral-have their fre-.

quencies distributed throughout the speech rangeyany means for effecting their suppression must necessarily be very sharply selective in order that the loss of speech energy in the suppressing device may not be excessive. In accordance with the present invention noise current suppression is effected by means of an acoustic wave filter so designed as to have narrow attenuation peaks at the harmonically related frequencies of the interfering noises, and to have substantially free transmission elsewhere in the voice frequency range.

, Power line currents may vary in frequency from time to time by a small amount, generally being held constant within one-half of one per cent. Consequently, the possible deviation of the frequency of any harmonic component is proportional to the frequency of that component,'and a satisfactory filter for the suppression of noise currents due to.

power line interference should therefore have attenuating bands wider at high frequencies than 'at low frequencies. If a filter be used whose bands are uniform in width, then, if the deviations of higher frequency'harmonics are adequately covered, the low frequency attenuation bands will be unnecessarily wide, and will remove more of the speech waves than is desirable. A feature of this invention is that the attenuation bands are graded 1nwidth in proportion to the frequency in such a manner as to effectually suppress all harmonics of the interfering current regardless of the ordinary variations of the frequency.

The filter in its preferred form is an acoustic device comprising a main sound wave con.- ducting tube having connected thereto at regular intervals, side branches consisting of tubes of uniform length. The side branches are made long enough to resonate at the lowest of the harmonically related interfering frequencies, thereby causing all of the interfering tones to be strongly suppressed. The sharp selective characteristic of the device,

and the graduation of the attenuating band rated into a telephone system by means of acoustical-electrical transducers, for example a pair of telephone receivers or by a transmitter at one end and a receiver at the other if one-way communication only is required.

' The invention can be understood from the following detailed description and reference to the drawings, in which Fi 1 illustrates an acoustic harmonic su ression filter system in accordance with t e invention; 7

Fig. 2 shows a transmission characteristic of a system like that shown in Fig. 1 in relation to the energy distribution characteristic of speech; I

Fig. 3 shows an adaptation of the invention in voice operated relay circuits of a telephone repeater.

Referring to Fig. 1, the system comprises a main sound channel, constituted by a pipe 10,

to which at equal intervals are connected at coupled at each end to telephone receivers 13 and 13' which are provided with binding posts 14, 15 and 14, 15 whereby. the device may be connected into an electrical line. This connection is made through impedance 'matching transformers 16 and 16' the pri- 17, 18'. The operation is as follows: The receiver 13 converts electrical waves from the line 17-18 into sound waves which are propa gated through the sound conduit 10 to the to each of the pairs of side branches is cumureceiver 13', where'they are reconverted into electrical waves and delivered to the line '17, 18'. The sound waves in passing through the sound conduit are modified by. the actionof the side branches. Each pair of side branches 4 acts as a unit, resonating at a series of harmonically related frequencies corresponding to the frequencies of the interfering cur-rents, thereby causing waves of these frequencies to be strongly attenuated, lhe attenuation due lative and by the use of a sufiicient number of pairs the attenuation may be made as great asmay be desired.

The propagation constant per section, designated P, of an acoustic conduit having side branches is expressed by the following formuthc conduits due to viscosity:

ZwL Z -8 where 2L is the length of the main channel between the junction points ofside branches;

S is the cross-sectional area ofthe mainlcnannel;

The acoustic impedance of a sound path is v here defined as'the ratio of the excess pressure intensityto thevolumetric flow per sec- 0nd over thecross-section of the path atfa given point. The characteristic acoustic impedance of a sound path of one square centimeter area, denoted by Z above, is sometimes referred to as the specific characteristic impedance of a medium. The characteristic impedance of a path of any givenarea is illustrated in Fig. '1, impedance Z of each pair of side branches is expressed by the fol lowing formula, which neglects wave dissipation g Wlere Zis the length ofeach of the side branch u es.

. Equation 1) refers to a symmetrical T- section of the filter including a side branch pair and a portion of length L of the main.

exist at frequencies for which conduit on each side. Preferabl the whole filter should consist ofia plurality of these symmetrical sections and, length of conduit between the endside branches and the rethe eflect of varying the lengths of the end connections is not serious when the filter com'prises several sections. By substituting in equation (1) the value of Z expressed in equation ('2), equation (1) becomes ZcoL I All-inspection of equation (3) shows that thereistheoretically infinite attenuation at w r 20:5 frequencies for wb1ch tan is zero,

occurs when is equal to 1r or a multiplef thereof; hence the frequencies ofina x'irnum attenuation are harmonically reated.

7 When cosh P lies between the limits 1 and 1, there is no attenuation of waves in the la which assumes no dissipation of waves in. ifillefiiimd these limits determine the edges or cutofisof bands of free transmission.

The lower cut-eds of the transmission bands occur when cosh P= +1 and from equation (3) are found to lie at frequencies for which occur when "cosh P=i and are found to @L V I Q g 7 60h (5) -The transmitting and the attenuating bands-alternate with each other throughout the. frequency scale, and in general, the

theoretical widths of the attenuation bands are somewhere near the widths of the transmittiug bands; but it has been discovered that I for practical purposes the effective widthsof the attenuating bands are much tween a lower cut-01f and-an attenuation peak, there is very little attenuation so that waves [are relatively freely transmitted less than their theoretical widths at the lower equal to Z dlvlded by the area. For the filter fr through the filter-. At higher frequencies,

. ZcuL however, as cos approaches zero, the

tenuation regions increase. When cos is e ual to zero, the cut-offs are spaced equidis nt from the attenuation peak and the e ective attenuating band width is widest. As the frequency further increases, it is found that the effective attenuating band width becomes narrower again; but in prac-' tice this narrowing ofthe attenuation bands at high frequency is usually unimportant sincethe filter canhe proportioned so that the narrow attenuation bands at high frequencies occur above the frequencies of importance.

'Some control can also be' exercised over the hand widths by means of the ratio S the larger this ratio, thevwider being the attenuating bands.

It should be noted that the invention is not limited to the use of side branches which 'exist in pairs. The operation would be the same if the side branches were single, openended tubes of length 21; for it can be shown that the impedance of the parallel combination of an open-ended and a closed-ended tube, each of length Z, is equal to the impedance of an open-ended tube of length 21. To design a filter to suppress harmonical- 36' ly related frequencies the widest effective attenuation bands should be situated at the highest frequency of importance, which in speech transmission lines is generally about 2000-2500 c. p. s., and in voice-operated re- 40 lay circuits is generally about the same, being dependent on the energy of the power line harmonics. Since the Widest efi'ective attenuation band occurs when cos 0,

the length, 2L, of the main conduit between side branches is determined from the relation the value of to being chosen to correspond to this highest frequency of importance. The length I of the side branches is determined from the relation Zwl which is the, condition for tan being when this frequency is placed high in the speech range. On the other hand, since the branchpaths must have resonance at the lowest frequency of the interfering waves,

their lengths are relatively large. The cross-sectional area, S of the main jpipe may be any convenient size and should be chosen to fit the telephone receivers. The side branch cross-section S can conveniently be made equal to S ,-although as explained above, the attenuation band widths can be increased or decreased by increasing or decreasing S The following example illustrates the design of a harmonic suppression system which was actually built to suppress the noise in a voice-operated relay circuit, due to a 6.0 c. p. 5. power line. The telephone receivers to be used are of a standard type and are adapted to be coupled to an acoustical device whose diameter at the neck is .7", which consequently must be diameter of the main sound .pipe. The highest harmonic of importance in this case is taken-to be 2100 c. p. s. so the widest attenuation band is to be placed at this frequency. In formula (6), then m is taken as 9m timcs'2100, and since the velocity of sound, 0 is 13,600" per second, the distance 2L between junctions is found to be 1.61 inches. Since the side branches must resonate waves of c. p. s. and their harmonics, the value of (o in formula (7) is taken as 211- times 60, and the value of .l is found to be 56.67. For convenience the cross-section S of the side branches is taken equal to S It is found that six sections of this filter are sufiicient to adequately suppress the power line harmonics. The total length of a six section filter is 1.61'X6=9.66" from which it is seen that the time of propagation through the filter is very small. The time of delay, designated T, is expressed by I where at is the number of. sections, and in the example considered has the value .00072 see.

Fig. 2 shows the relation between the at tenuation of the filter and the power in the voice; Curve a represents the overall attenuation of the filter system, including the receiver, of the above example. The attenuation is plotted along the axi'sof ordinates, the

unit of attenuation being the decibel, which is a logarithmic measure of the ratio of the wave power. A treatment ofthe decibel is given by W. H. Martin in an article in the Journal of the American Institute of Electrical Engineers,March 1929, page 223. It,

is seen that the bands of great-attenuation are very narrow at low frequencies, while at higher frequencies they are wider. Due to the fact that there is some dissipation of wave energy from viscosity in the pipes, the attenuation is never zero. The increased attenuation noted at the upper end of the frequency spectrum is the result of dissipation.

the voice energy exists in waves of compare-.- tively low frequency, and since the bands of great attenuation of the filter are very narrow at these frequencies, most of the voice energy is transmitted with comparative freedom. At the higher frequencies, around 2000c. p. s., where the attenuation'bands are vwidest, 'and less of the voice is transmitted,

there is not much energy in the voice anyway. But, since the attenuation bands are wider at the upper frequencies, they. are adapted to cover the harmonic noise fre' quency variation, which isgreater at *high than at low frequency. Although the attenuationband widths decrease again at fre-v q uencies above that for which cos g-=0,

which in this case is 2100 c. p. s., the amplitude of power lineharmonics are generally so small at these higher frequencies as to cause no trouble. 7

Fig. 3 illustrates a two-way telephone repeater equipped with echo-suppressor circuitsv in which harmonic suppression filters of this invention are utilized. The application of.

echo-suppressors in telephonerepeaters is well known and a treatment of the subject n is given by Clark and Mathes in the Journal rents before entering the relays aregmade to of the Americanlnstitute of Electrical Engineers. June 1925, pages. 6184326. The repeater illustrated is a well known type com prising two speech transmission paths joined at the ends by hybrid coils and 31. The I telephone lines which are connected by the ma$arise conditions would make des1rab1e repeater are represented by squares W and The squares designated N and N represent balancing networks. The upper path, which transmits in one direction only, from left to right, comprisesthree speech amplis fiers 45,46 and 47 and a delay network D.

The lower path comprises amplifiers 48, 49

and and transmits in the opposite direct1on. The echo suppressor circuit comprises two parallel circuits 32 and 33 bridged across .the two sides of the four-wire circuit, and a "of the repeater. circuit. The other end of the filter is connected tothe input of an amplifier detector 36" which operates a relay 37. The

' operationof the relay closespontact 38 thus lshortcirc-uiting the upper transmission path.

its filter system,-designated 39, isconnected to the upper transmission path instead of the 7 7 lower. The, filter system is Lconnected in tandem with an amplifier-detector 40 whose output is in series with two relays 41 and42. The operation of these relays closes contact- 43 .Which shortcircuits the lower transmission path andopens contact {.14 which removes the short circuit across the upper path.

Briefly, the operafion'is as follows: When there is no talking or other disturbance, con-' tacts 38 and 43 are open and contact 4-1 is closed. l/Vhen talking commences on line W, the voice currents travel. along the upper transmission path and asmall portion of the energy, entering circuit 33, operates relays 41 and 42, thus applying a short circuit across the lower transmission path. and removing the short circuit across the upper-path. There is then a direct path-for transmission to line 35 E; but BCllOQS'iCflDIlOlS return along the lower path because of-theshort' circuit there The purpose of the delay network I) in the upper path is to holdup the passage of the voice currents until relay 42 has had time. to operate.

When thetalking from lineW ceases, the relays return to normal. Then when line E commences talking, the voice currents travel through the lower-path and part of the energy, entering circuit 32, causes relay 37 to; operate thus putting-a short circuit across the input to circuit 33 which mightotherwise be set into operation by the echoes;

If the filters were not present might be operated by noise in the system, particularly power line noise. Since the curpass through filters, most of the power line lays.

the invention is probably'lnostapplicable in voice operated relay circuits there the relays energy is prevented from reachingthe. re- 7 the use of' the filter directly in the telephone line. Such a condition would .be the case where power line disturbances are so'severe.

as to impair the intelligibility of speech re- 1. The combination a telephone line subject to speech currents and relatively sustained interfering currents, of an acoustical wave filter for selectively suppressing the interfering currents, said filter comprising a main sound conduit and a plurality of side.

- branch tubes connectedto said main conduit at regular intervals, said side branch tubes having a uniform length of such a value that they resonate a series of harmonically related It Circuit 33 is'snnilar to circuit 32 except that frequencies corresponding to the frequenmu 9 at regular c1es of the sustained interfering waves, and the length of the portion of said main channel between side branches being small in comparison to the length of the side'branches.

v 2. In a voice operated relay circuit, acoustic means for-separating relatively sustained noise waves from speech waves, said means comprising a main wave conducting acoustical channel subject to both speech waves and sustained noise waves, there being connected to said main wave conductingchannel at regular intervals, acoustical side branch tubes having such a length as to be resonant at a series of harmonically related sustained noise waves present in the system, and the lengths of the portions of said main channel between the junction points of side branches being not greater than about one-quarter of the wave length of the highest noise fre- 20 'quency of importance, whereby saidvmeans is characterized by attenuation bands which are a narrow at low frequency and progressively increase in width up to said highest voice frequency of importance.

3. In combination, a voice transmission line, an acoustic filter and two acousticalelectrical translating devices, said filter comprising a main sound conduit having a plurality of side branch tubes connected thereto intervals, said side branches having a uniform length of such a value that they resonate aseries of harmonically related frequencies corresponding to the frequencies of the sustained interfering waves, said' main conduit being inserted between two portions of said transmission line and connected thereto by means of one of said translating devices at each end, the length of said main conduit between the junction points of side branches 40 being small whereby the attenuation bands,

which include the frequencies resonated by lengths of the side branch tubes being suchv that they resonate a series of harmonically related sustained noise waves present in the system and the length of said main wave conducting channel between the junction points of side branches being equal to one-quarter the wave length of'the highest noise harmonic to be effectively attenuated, whereby said means is characterized by attenuation bands which are narrow at low frequency and progressively increase in width at higher frequencies toa maximum at the highest noise harmonic to be attenuated.

In witness whereof, I hereunto subscribe my name this'27th da of June, 1929.

' I WA REN- P. MASON.

sively increase in width up to a frequem said side branches, are narrow at low frequency and progressively increase in width up to a high frequency 45 4. A filter systemi-fbr suppressing noise currents in a speechitrynsmission line, said system comprising aniwustic filter and two telephone receivers, one at each end of said filter, for connecting said filter into said 50 transmission line said filter comprising a eech side. branches connected'to said main sound con main sound channel, subject to both waves and sustained noise waves, an

duit at regular intervals, said side'branches being tubes resonant at a series of harmonical- 1y related sustained noise waves present in the system, the lengths of the portions of said main channel between the junction .points of side branches being small and the ratio of the 60 cross-section of the side branches to the crosssection of the main channel being small whereb said system is characterized by narrow je ective attenuation bands which include the frequencies at ,which said side branches are resonant, and the bands progres- 

